How to Fast: A Guide to Fasting Methods and Safety

Medically Reviewed By Adam Bernstein, MD, ScD
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Discussing a fasting plan with your doctor and learning how to fast safely may be beneficial for some people. While some fasting plans have evidence-backed short-term benefits, others may do more harm than good. Humans have been fasting for health reasons for centuries. Some fasting methods are said to improve your metabolic health, help you lose weight, or “detox.” Still, fasting isn’t for everyone, and science doesn’t support all of these claims.

Read on to learn more about how to fast, safety tips, and who may want to avoid fasting. 

What is fasting?

Vegetables sitting on a plate with a clock face on it
Juan Moyano/Stocksy United

Fasting refers to refraining from eating food, and sometimes certain beverages, for some time. Most people fast when they sleep, unless they get up for a snack. Some blood tests require you to fast for at least 8 hours beforehand. 

There are many fasting methods you can choose from. Some set standards for the times of day you can eat. Others simply focus on what you can or cannot consume during the fast. 

Why do people fast?

While periodic fasting is important in some religions, many people typically fast to achieve a health goal. Although fasting may not achieve all types of health goals, people often fast with the hopes of: 

  • losing weight
  • “detoxing”
  • managing blood sugar in type 2 diabetes
  • improving metabolic health
  • lowering cholesterol

What are some different fasting methods?

Fasting methods vary based on schedule and nutrient restrictions. 

Intermittent fasting methods

Intermittent fasting involves alternating between fasting and eating on a set schedule. According to the National Institute on Aging (NIA), hundreds of studies on animals and humans and intermittent fasting suggest this method may have numerous health benefits. Some of these include benefits for people with: 

  • obesity
  • diabetes
  • cardiovascular disease
  • cancers 
  • neurological disorders

However, the NIA also notes that while studies may point to short-term benefits, potential long-term benefits require more study.

There are several types of intermittent fasting methods you can follow. For example:

  • The 16:8 method limits your eating window to 8 hours a day and expands your fasting window to 16 hours.
  • The Eat-Stop-Eat intermittent fasting method requires fasting for a full day once or twice a week.
  • Alternate-day fasting requires you to fast every other day.
  • The One Meal A Day or OMAD diet requires you to consume all daily calories in a short time window or in one meal. You can consume beverages that don’t contain calories in the fasting window.

The previous intermittent fasting methods do not recommend calorie restriction during the eating methods. However, you may consume fewer calories due to the restrictive eating time.

However, the 5:2 intermittent fasting method does recommend some calorie restriction. On this diet, the accepted practice is to consume only about 500 calories on two back-to-back days a week. Then you can eat as you typically would on the other days of the week.

Learn about 8 health benefits of intermittent fasting.

Nutrient-restrictive cleanses 

Other fasting plans limit the type of foods and beverages you can consume for a period, like juice or bone broth fasts.

A 2018 study suggests that water-only fasts can be safe, with a low risk of serious adverse effects, for two or more days if done in a medically supervised setting. However, while proponents of these diets say these fasts can “cleanse” your body of toxins and help you lose weight, more research is needed.

The National Center for Complementary and Integrative Health warns that few studies support the benefits of cleanses, and any resulting weight loss is not typically long lasting.

Also, depriving your body of a variety of nutrients can be harmful to your health.

Learn more about types of cleanses and their potential risks.

How can you fast safely?

If you do decide you want to fast, it’s important to keep a few safety protocols in mind: 

  • Always talk with your doctor before beginning any fast. 
  • Make sure to stay well-hydrated during your fast. 
  • Get adequate rest during your fast, and do not engage in strenuous exercise. 
  • Never use a “detox” or “cleansing product” without your doctor’s approval. The Food and Drug Administration (FDA) and Federal Trade Commission have discovered many of these products contain ingredients that could be harmful
  • Talk with your doctor about how to safely take your medications around a fasting schedule. You may need to temporarily break a fast to take prescriptions that require food intake. 

Are there certain groups who should not fast?

If you are underweight or have a history of disordered eating, you should avoid fasting or restrictive diets.

You also should not fast if you are pregnant or nursing. Fasting can prevent a growing fetus from receiving nutrients. Children, adolescents, and older adults should also not fast.

In addition, intermittent fasting may cause side effects such as muscle loss or an electrolyte imbalance. You should discuss fasting with your doctor if you’re taking medications that may make you more prone to an electrolyte imbalance, such as certain blood pressure medications or diuretics.

Other frequently asked questions

These are a few other common questions about fasting. Adam Bernstein, M.D., ScD, reviewed the answers.

What is the proper way to fast?

Different fasting methods set down different ways to fast. For example, the 16:8 method requires you to restrict eating to an 8-hour window while fasting for the remaining 16 hours. Not everyone should fast, so always talk with your doctor before trying any fasting method.

Does sleeping count as fasting?

Yes, sleeping counts as fasting. Fasting is defined as refraining from eating for a period of time.

Summary

Intermittent fasting may be an option for some people looking for certain health benefits, like improvements in obesity, diabetes, or other conditions. If you are fasting, stay hydrated and avoid intense workouts. 

When choosing a fasting method, keep in mind that intermittent fasting has more well-researched benefits than restrictive cleansing fasts. Some people may feel better on one type of fasting schedule than another.

If you want to fast for a long period or try a water-only fast, you should receive medical monitoring. Also, avoid fasting if you have certain medical conditions, such as a history of disordered eating or if you are pregnant or nursing.

Talk with your doctor before starting any fast.

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Medical Reviewer: Adam Bernstein, MD, ScD
Last Review Date: 2022 Nov 29
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THIS TOOL DOES NOT PROVIDE MEDICAL ADVICE. It is intended for informational purposes only. It is not a substitute for professional medical advice, diagnosis or treatment. Never ignore professional medical advice in seeking treatment because of something you have read on the site. If you think you may have a medical emergency, immediately call your doctor or dial 911.
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